- Email: [email protected]
Comparison of two agar dilution methods and three agar diffusion methods, including the Etest, for antibiotic susceptibility testing of thermophilic Campylobacter species
580
C l i n i c a l M i c r o b i o l o g y a n d Infection. V o l u m e 5 N u m b e r 9, S e p t e m b e r 1999
Comparison of two agar dilution methods and three agar diffusion methods, including the Etest, for antibiotic susceptibility testing of thermophilic Campylobacter species Clin Microhiof Infect 1999; 5: 580-584
J0tgen Engberg I", Sigrid Andersen 2, Robert Skov Peter Gerner-Smidt
',Frank Moller Aarestrup
and
IDepartment of Gastrointestinal Infections, Division of Microbiology, Statens Serum Institut, Artillerivej 5, 2300 Copenhagen S, Copenhagen; 2Danish Veterinary and Food Administration, Copenhagen; 3Danish Veterinary Laboratory, Copenhagen, Denmark *Tel: +45 3268 3648 Fax: +45 3268 3873, E-mail: [email protected] Accepted 8 April 1999
For Campylobacter spp., no internationally accepted criteria for susceptibility testing including assessments of breakpoints for susceptible versus resistant isolates are available. Therefore, routine susceptibility testing of Campylobacter in the clinical microbiology laboratory is often not performed. For therapeutic agents, the breakpoints established for aerobic bacteria are often used. However, the extrapolation of such breakpoints to other bacteria like campylobacters, which require special growth conditions, may be questioned. Campylobacteriosis is considered to be a zoonotic disease, and emergence of antimicrobial resistance in enteric Campylobacter spp., due to the use of antimicrobial agents in husbandry, is a matter of concern [l-41. In Denmark, more than 80% of the cases of campylobacteriosis are estimated to be domestically acquired from food and to a lesser extent drinking water [5]. In order to survey the antimicrobial susceptibility patterns of thermophilic Campylobacter spp. isolated from food animals, food of animal origin and humans in different laboratories, the same standardized or intercalibrated methods should be applied. MIC determinations are normally considered to be the standard for susceptibilitytesting. However, a variety of Werent methods, including a s i o n tests, are routinely used in clinical laboratories. Thus, comparative studies on the performance of diffusion testing procedures versus MIC determinations are needed. In the present study, we have compared the results obtained by the methods used in four clinical, veterinary and food microbiology reference laboratories in Denmark. In total, 24 human clinical isolates, 24 isolates from cattle, 24 isolates from broilers and 24 isolates from swine were included in the study (59 Campylobacter jejuni, 36 C. coli and one C. lari). The reference strains
CCUG 26277 (C.jejuni] and CCUG 11283 (C. coli], were included. Isolates from humans originated f k m stools of clinical cases of diarrhea submitted to Statens Serum Institut. Isolates from food animals originated from fecal samples taken at the time of slaughter from healthy animals and submitted to the Danish Veterinary Laboratory as part of the surveillance scheme for antimicrobial resistance in Denmark. The isolates were randomly selected from a stock culture collection known to have a variety of susceptibility characteristics with respect to the antimicrobial agents included in the present study. The isolates had been identified to the species level on the basis of standardized conventional methods: morphology, motility, catalase, oxidase, indoxyl acetate hydrolysis, hippurate hydrolysis, and susceptibility to nalidixic acid and cephalothin [6]. Subcultures from a single colony for each isolate were performed and isolates were distributed to the participating laboratories frozen in filtered serum broth with 10% glycerol. Apart from method A, all isolates were tested twice on two different days to allow for day-to-day variation, and results therefore consist of an average of two values. For quality control, the following reference strains were included: Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853) and Enterococcus faecalis (ATCC 29212). The following antimicrobial agents were tested nalidixic acid, erythromycin, streptomycin and tetracycline. For the agar dilution methods, the antibiotics were bought &om Sigma Chemical Company, St Louis, MO, USA. Study C: AB Biodisk, Solna, Sweden. Studes D and E: Rosco, Taastrup, Denmark.
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AGAR DILUTION WITH YEAST-ENRICHED DANISH BLOOD AGAR METHOD A
The dilution ranges were as follows: erythromycin and tetracycline, 0.03 1-256 mg/L; nalidxic acid, 1-256 mg/L; and streptomycin, 0.5-256 mg/L. MIC determinations were performed by the agar dilution method with a yeast-enriched 5% Danish blood agar plate (SSI Diagnostica, Hillercld, Denmark). All MIC plates were inoculated with 1 pL of approximately lo7 CFU/mL with a Denley multipoint inoculator (Denley Instruments, Billingshurst, UK), resulting in lo4 CFU/spot. The plates were incubated for 24 h at 37°C in a microaerobic atmosphere (5% 0 2 , 5% COz, 90% Nz). A pilot study (data not presented) had shown that sufficiently good growth of thermophilic Campylobacter spp. could be achieved on the medium within 24 h of incubation. The MIC was defined as the lowest con-centration producing three or fewer colonies. AGAR DILUTION WITH MUELLER-HINTON METHOD B
H AGAR
The dilution ranges were as follows: erythromycin and tetracycline, 0.25-32 mg/L; nalidixic acid and streptomycin, 1-128 mg/L. MIC determinations were performed by the agar dilution method with MuellerHinton I1 agar (Becton Dickinson Microbiology Systems, Cockeyville, MD, USA.) supplemented with 5% bovine blood. All MIC plates were inoculated with approximately lo4 CFU following the procedure of Tenover et a1 [7]. The plates were incubated for 48 h at 37°C in a micro-aerobic atmosphere (6%0 2 , 7% COz, 7% H2, 80% N2). The MIC was defined as the lowest concentration producing no visible growth. ETEST WITH YEAST-ENRICHED DANISH BLOOD AGAR: METHOD C
This method was performed by the application of Etest strips on a swab-inoculated, yeast-enriched 5% Danish blood agar dish (SSI Diagnostica). Concentrations of all antimicrobial agents ranged &om 0.016 mg/L to 256 mg/L. To diminish moisture in the dishes, they were allowed to dry with lids ajar for 15 min before use. The inoculum, obtained from a 24-h-old culture, was standardized according to the direct colony suspension method described by the NCCLS [8], but with a turbidity equal to a no. 6 McFarland standard to achieve satisfactory growth after 24 h of incubation. Two drops of this suspension were pipetted and evenly distributed over the entire surface of a 140-mm dried agar dish using a bent glass rod. After application of the strips with forceps, each dish was incubated at 37°C in a
micro-aerobic atmosphere (5% 0 2 , 5% COz, 90% N2). with no more than 10 dishes stacked together. MICs were determined after 24 h of incubation by reading the corresponding value listed on the Etest strip scale, where the eliptical zone of inhibition intersected the strip, Because of observed swarm into areas of inhibited growth, the veil of swarming growth was ignored. AGAR DIFFUSIONWITH YEAST-ENRICHED DANISH BLOOD AGAR: METHOD D
This method was performed by the application of antibiotic tablets on the same type of plate as in method C. To diminish moisture in the dishes, they were allowed to dry with lids ajar for 15 min before use. The antibiotic contents of the tablets were as follows: tetracycline 80 pg, erythromycin 78 pg, streptomycin 100 pg and nalidixic acid 130 pg. The inocula were the same and prepared in the same way as for method C. After application of the tablets by a tablet dispenser, each dish was incubated at 37°C in a micro-aerobic atmosphere (5% 0 2 , 5% CO2, 90% Nz). Measurement of zone diameters was made from the zone edge of complete inhibition. Because of observed swarm into areas of inhibited growth, the veil of swarming growth was ignored. AGAR DIFFUSIONWITH BLOOD AGAR BASE 2 (OXOID): METHOD E
This method is based upon embedding of a bacterial inoculum in agar followed by application of the same antibiotic tablets as in method D to the agar surface. An inoculum of 0.3 mL (10' CFU/mL) was prepared from three loops of fresh colony material in 2 mL of 0.9% NaC1, and mixed thoroughly with 60 g of autoclaved and liquid (48.0+1.0°C) blood agar base No. 2 (Oxoid, Basingstoke, UK) in sterile 140-mm Petri dishes. After the agar had solidified, the dishes were dried for 30 min at room temperature in a laminar airflow bench. The four antibiotic tablets were applied on the surface of each dish and incubated at 420°C for 48 h in a micro-aerobic atmosphere of approximately 6% 0 2 , 7 % COz, 7%Hz, 80% Nz. Interpretive criteria for breakpoints were determined by comparison of the distribution of the population of MICs for methods A-C and zones of inhibition for methods D and E. For the clinically relevant agent, erythromycin, a regression line between methods A and D was produced. The distribution of the susceptibilities of all four antibiotics with all five methods was bimodal, with breakpoints at clinically achievable levels. The percentage of sensitive isolates was virtually the same for
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each agent, regardless of the method: nalidixic acid 92-94%, erythromycin 82-83%, streptomycin 85%and tetracycline 98%. As an example for the distribution of MICs the data for method A is presented in Table 1. With the tentative breakpoints listed in Table 2, there was complete agreement between the three Table 1 The distribution of the population of MICs for method A for the four antimicrobial agents MIC 0.031 0.0625 0.125 0.25 0.5 1 2 4 8 16 32 64 128 256 >256
Nalidixic acid
Erythromycin Tetracycline Streptomycin 0 0 0 3
18 35 15 9 1 0 0 1 2 0 14
0 5 46 28 11 2 0 0 2 4
0 0 11 36 38 9 1 1 0 0 0 0 2 0 0
0 9 38 17 14 5 0 2 2 3
8
MIC methods with regard to separating isolates into susceptible and resistant populations when tested for all four antimicrobial agents. However, for nahdixic acid, two isolates were resistant according to MIC methods (MIC ranges 32-64 mg/L) but had zones of inhbition between 26 and 31 mm with the two tablet diffusion methods. For erythromycin, the regression line for zone versus MIC for methods A and D showed good correlation (Figure 1, r2=0.87). Within the group of susceptible isolates, the distribution of dfferences was assessed for each antimicrobial agent (Table 3). Because of differences in ranges between MIC methods, out-of-range values are not included in the calculations presented in Table 3. There is at present no internationally accepted standard procedure for susceptibility testing of Camp+ bacter, and a variety of methods are used. In the present study, we have compared five hfferent methods in order to assess the comparability of results obtained in different laboratories. There was agreement concerning the separation of nahdxic acid MICs into susceptible and resistant groups. For this agent, the Etest had a tendency to produce lower values compared to the two agar ddution methods (Table 3). However, these discrepancies in dilutions were mostly some distance from the breakpoint and did not cause problems with
MIC, mg/L
1024 1 512 -
o*o 0
256 128 64 32 16 84-
210.5 0.25 -
0
0
I
I
I
I
0
I
Figure 1 Erythromycin, regression line for zone versus MIC (methods A and D). Asterix indicates 12 values. Regression line: y = - 1 2 . 7 6 ~+ 569,7; r2 = 0.87.
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respect to the overall purpose of the present study: the separation of isolates into susceptible and resistant groups. The proper separation of isolates is of major importance, as nalidixic acid is used to identify Cutnpylobucter to the species level [6], even though at present there is no zone diameter criterion for defining susceptibility to this agent. Resistance of C. jejuni and C. coli to quinolones is emerging in many countries [9-121. This will cause problems with species identification when this is performed solely by phenotypic tests, the most frequently used method for differentiating Cumpylobucter spp. in routine laboratories [13,14]. More noteworthy is the fact that nalidixic acid susceptibility is a marker for C. jejuni and C. coli ciprofloxacin susceptibility; the nalidixic acid-susceptible strains are susceptible to ciprofloxacin, while most of the resistant ones are resistant to ciprofloxacin [lo,151. A high level of nalidixic acid is therefore indicative of resistance to ciprofloxacin, an important drug of choice for the treatment of severe invasive campylobacteriosis [16]. The level of nalidixic acid resistance in C. jejuni isolated from humans in Denmark has previously been assessed by methods A and D, and is approximately Table 2 Tentative breakpoints for the four antimicrobial agents Antimicrobial
MIC methods
Tablet methods
(mg4
(mm)
SS16; R 2 3 2 SS8; R 2 6 4 S116; R S 3 2 S14; R 2 3 2
S226; R 1 1 7 S227; R 1 2 0 SS31; R 1 2 5 S225; R 1 1 8
agents Nalidixic acid Erythromycin Streptomycin Tetracycline
S, susceptible; R , resistant.
10-15% [17,18]. This moderate level can be attributed to the infrequent use of antibiotics for the treatment of diarrhea in Denmark and the fact that the veterinary use of quinolones for therapy of infections has so far been relatively limited. In the present study, all methods agreed in categorizing the naturally resistant C. luri isolate as resistant to nalidixic acid. The general tendency of the Etest to produce lower values than the agar dilution methods was more pronounced for erythromycin than for nalidixic acid. The tendency was mainly seen at very low MICs, and therefore did not interfere with the interpretation of results. Resistance to erythromycin is more frequent in C. coli than in C. jejuni [17,18]. In the present study, 42% of C. coli and 3% of C.jejuni isolates were resistant to erythromycin. The agent is considered to be the drug of first choice when simple rehydration and electrolyte replacement is not sufficient for the treatment of Campylobacter enteritis [16,19]. The reliability of the reported susceptibility testing on this agent is therefore of major clinical importance. Our regression line in Figure 1 indicates that the tablet method may constitute a simple, inexpensive and accurate means of susceptibility testing of Cumpylobucter for erythromycin. For streptomycin and tetracycline, there was complete agreement between all five methods. For both agents, a large number of isolates had MICs out of range, so the calculations in Table 3 are based upon moderate numbers, 16 and 25 respectively. As shown in Table 3 , method B resulted in lower values than methods A and C when tested for streptomycin. Interpretation was clearcut for tetracycline because of full agreement between methods and MICs and zones of inhibitions clearly away from the breakpoint. Correlation analysis with this antibiotic is weakened by
Table 3 Distribution of differencesb in MICs for susceptible isolates (%) Antimicrobial agent
~
a
N"
~~
Methods
< -2
-3
-1
0
+1
+2
>+2
~
N inmcates number of susceptible isolates for whch comparison was performed
Zero indlcates percentage of isolates for which MICs are identical within one ddutlon; - 1 and further.
+1 etc inmcate? 1 log2 ddution dlfference
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the low occurrence of resistance in the isolates in the study. The tetracyline-resistant C. jejuni reference strain was correctly categorized as resistant by all five methods, indicating that the low occurrence of resistance to tetracycline is correct and not methodological. Low levels of resistance to tetracycline in Denmark have previously been shown for both C. jejuni and C. coli [17,181. In conclusion, the Etest had a tendency to produce lower values compared to agar ddutions when tested against nahdixic acid and erythromycin. In contrast, method B resulted in lower values compared to the two other MIC methods when tested against streptomycin. However, both methods were able to separate isolates unequivocally into susceptible and resistant groups, and the findings are therefore of minor significance. Even though monitoring of the resistance for surveillance purposes is performed in different laboratories and with different techniques, our study indicates that reliable results can be achieved for comparison. However, the interpretive criteria that we have suggested must be considered tentative because of the low level of resistance to some antibiotics among a moderate number of isolates at study. It would be appropriate to evaluate these tentative criteria in a larger multicenter study, including a variety of antibiotics that have clinical, diagnostic or epidemiologic relevance. Acknowledgments
We are grateful to Annette Amdt, Frank Hansen, Bente Hansen, RenC Hendriksen and Anette Gggsig Christensen for technical assistance and Simoco ApS, Lyngby, Denmark for co-financing of Etest strips. This study was a part of the Danish Integrated Antimicrobial Resistance Monitoring and Research Program (DANMAP), conducted in collaboration between Statens Serum Institut, the Danish Veterinary and Food Administration and the Danish Veterinary Laboratory, and funded jointly by the Danish Ministry of Health and the Danish Ministry of Food, Agriculture, and Fisheries. References 1. Piddock LJV Quinolone resistance and Campylobacter spp. J Antimicrob Chemother 1995; 36: 891-98. 2. Gaunt PN, Piddock LJV Ciprofloxacin resistant Campylobactn spp. in humans: an epidemiological and laboratory study. J Antimicrob Chemother 1996; 37: 747-57. 3. World Health Orgdnization. The medical impact of the use of antimicrobials in food animals. Report of a WHO Meeting, Berlin.13-17 October 1997. Geneva: WHO, 1997.
4. World Health Organization.Major gaps in research on antibiotic resistance need filling. Press Release WH0/46, WHO, 1998. 5. Neimann J, Engberg J, M~lbakK, Wegener HC. Risk factors associated with sporadic campylobacreriosis in Denmark. In: Proceedings of the 4th World Congress on Foodborne Infections and Intoxications, Berlin: Federal Institute for Health Protection of Consumers and Veterinary Medicine, 1998: 29&303. 6. Barrow GI, Feltham RKA. Cowan and Steel's manual for the identification of medical bacteria, 3rd edn. Cambridge: Cambridge University Press, 1993. 7. Tenover FC, Baker CN, Fennell CL, Ryan CA. Antimicrobial resistance in Campylobacter species. In Nachamkin I, Blaser MJ, Thomkins LS, eds. Campylobacterjejuni: current status and future trends. Washington DC: American Society for Microbiology, 1992; 6G73. 8. National Committee for Clinical Laboratory Standards. Performance standards for antimicrobial disk susceptibility tests, 6th edn. Approved Standard. NCCLS document M2-M6. Wayne, PA. NCCLS, 1997. 9. Sjogren E, Lindblom GB, Kaijser B. Nodoxacin resistance in Campylobacter jejuni and Campylobacter coli isolates h m Swedish patients. J Antimicrob Chemother 1997; 4 0 257-61. 10. Rautelin H, Renkonen 0-V, Kosunen TU. Emergence of fluoroquinolone-resistance in Campylobacter jejuni and Campylobacter coli in subjects fkom Finland. Antimicrob Agents Chemother 1991; 35: 2065-9. 11. Sinchez R, Fernindez-Baca V, Diaz MD, Muiios P, RodriguezCreiems M, Bouza E. Evolution of susceptibilities of Campylobacter spp. to quinolones and macrolides. Antimicrob Agents Chemother 1994; 38: 1879-82. 12. Velizquez JB, Jimenez A, Chombn B, Villa TG. Incidence and transmission of antibiotic resistance in Campylobacter jejuni and Campylobacter coli. J Antimicrob Chemother 1995; 35: 173-8. 13 On SLW. Identification methods for campylobacters, helicobacters, and related organisms. Clin Microbiol Rev 1996; 9: 405-22. 14 Vandamme P, Goossens H. Taxonomy of Campylobacter, Arcobacter, and Helimbacter. a review. Z Bakteriol 1992; 276: 447-72. 15 Endtz PH, Ruijs GJ, van IUingern B, Jansen WH, van der Reyden T, Mouton RF! Quinolone resistance in Campylobacter isolated from man and poultry following the introduction of fluoroquinolones in veterinary medicine. J Antimicrob Chemother 1991; 27: 199-208. 16 Skirrow MB, Blaser MJ. Campylobatterjejuni. In Blaser MJ, Smith eds. Infections of PD, Ravdin JI. Greenberg HB, Guerrant IU, the gastrointestinal tract. New York: Raven Press, 1995: 825-48. 17. Aarestrup FM, Nielsen EM, Madsen M, Engberg J. Antimicrobial susceptibility patterns of thermophilic Campylobacter spp. &om humans, pigs, cattle and broilers in Denmark. Antimicrob Agents Chemother 1997; 41: 2244-50. 18. Anon. In: Consumption of antimicrobial agents and occurrence of antimicrobial resistance in bacteria fkom food animals, food and humans in Denmark, 1997. Danish Integrated Antimicrobial Resistance Monitoring and Research Programme (DANMAP), Copenhagen: Danish Zoonosis Center, 1998. 19. Taylor DE. Antimicrobial resistance of Campylobacter jejuni and Campylobacter coli to tetracycline, chloramphenicol and erythromycin. In Nachamkin I, Blaser MJ, Thomkins LS, eds. Campylobacter jejuni: current status and future trends. Washington DC: American Society for Microbiology, 1992 74-86.
C l i n i c a l M i c r o b i o l o g y a n d Infection. V o l u m e 5 N u m b e r 9, S e p t e m b e r 1999
Comparison of two agar dilution methods and three agar diffusion methods, including the Etest, for antibiotic susceptibility testing of thermophilic Campylobacter species Clin Microhiof Infect 1999; 5: 580-584
J0tgen Engberg I", Sigrid Andersen 2, Robert Skov Peter Gerner-Smidt
',Frank Moller Aarestrup
and
IDepartment of Gastrointestinal Infections, Division of Microbiology, Statens Serum Institut, Artillerivej 5, 2300 Copenhagen S, Copenhagen; 2Danish Veterinary and Food Administration, Copenhagen; 3Danish Veterinary Laboratory, Copenhagen, Denmark *Tel: +45 3268 3648 Fax: +45 3268 3873, E-mail: [email protected] Accepted 8 April 1999
For Campylobacter spp., no internationally accepted criteria for susceptibility testing including assessments of breakpoints for susceptible versus resistant isolates are available. Therefore, routine susceptibility testing of Campylobacter in the clinical microbiology laboratory is often not performed. For therapeutic agents, the breakpoints established for aerobic bacteria are often used. However, the extrapolation of such breakpoints to other bacteria like campylobacters, which require special growth conditions, may be questioned. Campylobacteriosis is considered to be a zoonotic disease, and emergence of antimicrobial resistance in enteric Campylobacter spp., due to the use of antimicrobial agents in husbandry, is a matter of concern [l-41. In Denmark, more than 80% of the cases of campylobacteriosis are estimated to be domestically acquired from food and to a lesser extent drinking water [5]. In order to survey the antimicrobial susceptibility patterns of thermophilic Campylobacter spp. isolated from food animals, food of animal origin and humans in different laboratories, the same standardized or intercalibrated methods should be applied. MIC determinations are normally considered to be the standard for susceptibilitytesting. However, a variety of Werent methods, including a s i o n tests, are routinely used in clinical laboratories. Thus, comparative studies on the performance of diffusion testing procedures versus MIC determinations are needed. In the present study, we have compared the results obtained by the methods used in four clinical, veterinary and food microbiology reference laboratories in Denmark. In total, 24 human clinical isolates, 24 isolates from cattle, 24 isolates from broilers and 24 isolates from swine were included in the study (59 Campylobacter jejuni, 36 C. coli and one C. lari). The reference strains
CCUG 26277 (C.jejuni] and CCUG 11283 (C. coli], were included. Isolates from humans originated f k m stools of clinical cases of diarrhea submitted to Statens Serum Institut. Isolates from food animals originated from fecal samples taken at the time of slaughter from healthy animals and submitted to the Danish Veterinary Laboratory as part of the surveillance scheme for antimicrobial resistance in Denmark. The isolates were randomly selected from a stock culture collection known to have a variety of susceptibility characteristics with respect to the antimicrobial agents included in the present study. The isolates had been identified to the species level on the basis of standardized conventional methods: morphology, motility, catalase, oxidase, indoxyl acetate hydrolysis, hippurate hydrolysis, and susceptibility to nalidixic acid and cephalothin [6]. Subcultures from a single colony for each isolate were performed and isolates were distributed to the participating laboratories frozen in filtered serum broth with 10% glycerol. Apart from method A, all isolates were tested twice on two different days to allow for day-to-day variation, and results therefore consist of an average of two values. For quality control, the following reference strains were included: Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853) and Enterococcus faecalis (ATCC 29212). The following antimicrobial agents were tested nalidixic acid, erythromycin, streptomycin and tetracycline. For the agar dilution methods, the antibiotics were bought &om Sigma Chemical Company, St Louis, MO, USA. Study C: AB Biodisk, Solna, Sweden. Studes D and E: Rosco, Taastrup, Denmark.
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AGAR DILUTION WITH YEAST-ENRICHED DANISH BLOOD AGAR METHOD A
The dilution ranges were as follows: erythromycin and tetracycline, 0.03 1-256 mg/L; nalidxic acid, 1-256 mg/L; and streptomycin, 0.5-256 mg/L. MIC determinations were performed by the agar dilution method with a yeast-enriched 5% Danish blood agar plate (SSI Diagnostica, Hillercld, Denmark). All MIC plates were inoculated with 1 pL of approximately lo7 CFU/mL with a Denley multipoint inoculator (Denley Instruments, Billingshurst, UK), resulting in lo4 CFU/spot. The plates were incubated for 24 h at 37°C in a microaerobic atmosphere (5% 0 2 , 5% COz, 90% Nz). A pilot study (data not presented) had shown that sufficiently good growth of thermophilic Campylobacter spp. could be achieved on the medium within 24 h of incubation. The MIC was defined as the lowest con-centration producing three or fewer colonies. AGAR DILUTION WITH MUELLER-HINTON METHOD B
H AGAR
The dilution ranges were as follows: erythromycin and tetracycline, 0.25-32 mg/L; nalidixic acid and streptomycin, 1-128 mg/L. MIC determinations were performed by the agar dilution method with MuellerHinton I1 agar (Becton Dickinson Microbiology Systems, Cockeyville, MD, USA.) supplemented with 5% bovine blood. All MIC plates were inoculated with approximately lo4 CFU following the procedure of Tenover et a1 [7]. The plates were incubated for 48 h at 37°C in a micro-aerobic atmosphere (6%0 2 , 7% COz, 7% H2, 80% N2). The MIC was defined as the lowest concentration producing no visible growth. ETEST WITH YEAST-ENRICHED DANISH BLOOD AGAR: METHOD C
This method was performed by the application of Etest strips on a swab-inoculated, yeast-enriched 5% Danish blood agar dish (SSI Diagnostica). Concentrations of all antimicrobial agents ranged &om 0.016 mg/L to 256 mg/L. To diminish moisture in the dishes, they were allowed to dry with lids ajar for 15 min before use. The inoculum, obtained from a 24-h-old culture, was standardized according to the direct colony suspension method described by the NCCLS [8], but with a turbidity equal to a no. 6 McFarland standard to achieve satisfactory growth after 24 h of incubation. Two drops of this suspension were pipetted and evenly distributed over the entire surface of a 140-mm dried agar dish using a bent glass rod. After application of the strips with forceps, each dish was incubated at 37°C in a
micro-aerobic atmosphere (5% 0 2 , 5% COz, 90% N2). with no more than 10 dishes stacked together. MICs were determined after 24 h of incubation by reading the corresponding value listed on the Etest strip scale, where the eliptical zone of inhibition intersected the strip, Because of observed swarm into areas of inhibited growth, the veil of swarming growth was ignored. AGAR DIFFUSIONWITH YEAST-ENRICHED DANISH BLOOD AGAR: METHOD D
This method was performed by the application of antibiotic tablets on the same type of plate as in method C. To diminish moisture in the dishes, they were allowed to dry with lids ajar for 15 min before use. The antibiotic contents of the tablets were as follows: tetracycline 80 pg, erythromycin 78 pg, streptomycin 100 pg and nalidixic acid 130 pg. The inocula were the same and prepared in the same way as for method C. After application of the tablets by a tablet dispenser, each dish was incubated at 37°C in a micro-aerobic atmosphere (5% 0 2 , 5% CO2, 90% Nz). Measurement of zone diameters was made from the zone edge of complete inhibition. Because of observed swarm into areas of inhibited growth, the veil of swarming growth was ignored. AGAR DIFFUSIONWITH BLOOD AGAR BASE 2 (OXOID): METHOD E
This method is based upon embedding of a bacterial inoculum in agar followed by application of the same antibiotic tablets as in method D to the agar surface. An inoculum of 0.3 mL (10' CFU/mL) was prepared from three loops of fresh colony material in 2 mL of 0.9% NaC1, and mixed thoroughly with 60 g of autoclaved and liquid (48.0+1.0°C) blood agar base No. 2 (Oxoid, Basingstoke, UK) in sterile 140-mm Petri dishes. After the agar had solidified, the dishes were dried for 30 min at room temperature in a laminar airflow bench. The four antibiotic tablets were applied on the surface of each dish and incubated at 420°C for 48 h in a micro-aerobic atmosphere of approximately 6% 0 2 , 7 % COz, 7%Hz, 80% Nz. Interpretive criteria for breakpoints were determined by comparison of the distribution of the population of MICs for methods A-C and zones of inhibition for methods D and E. For the clinically relevant agent, erythromycin, a regression line between methods A and D was produced. The distribution of the susceptibilities of all four antibiotics with all five methods was bimodal, with breakpoints at clinically achievable levels. The percentage of sensitive isolates was virtually the same for
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each agent, regardless of the method: nalidixic acid 92-94%, erythromycin 82-83%, streptomycin 85%and tetracycline 98%. As an example for the distribution of MICs the data for method A is presented in Table 1. With the tentative breakpoints listed in Table 2, there was complete agreement between the three Table 1 The distribution of the population of MICs for method A for the four antimicrobial agents MIC 0.031 0.0625 0.125 0.25 0.5 1 2 4 8 16 32 64 128 256 >256
Nalidixic acid
Erythromycin Tetracycline Streptomycin 0 0 0 3
18 35 15 9 1 0 0 1 2 0 14
0 5 46 28 11 2 0 0 2 4
0 0 11 36 38 9 1 1 0 0 0 0 2 0 0
0 9 38 17 14 5 0 2 2 3
8
MIC methods with regard to separating isolates into susceptible and resistant populations when tested for all four antimicrobial agents. However, for nahdixic acid, two isolates were resistant according to MIC methods (MIC ranges 32-64 mg/L) but had zones of inhbition between 26 and 31 mm with the two tablet diffusion methods. For erythromycin, the regression line for zone versus MIC for methods A and D showed good correlation (Figure 1, r2=0.87). Within the group of susceptible isolates, the distribution of dfferences was assessed for each antimicrobial agent (Table 3). Because of differences in ranges between MIC methods, out-of-range values are not included in the calculations presented in Table 3. There is at present no internationally accepted standard procedure for susceptibility testing of Camp+ bacter, and a variety of methods are used. In the present study, we have compared five hfferent methods in order to assess the comparability of results obtained in different laboratories. There was agreement concerning the separation of nahdxic acid MICs into susceptible and resistant groups. For this agent, the Etest had a tendency to produce lower values compared to the two agar ddution methods (Table 3). However, these discrepancies in dilutions were mostly some distance from the breakpoint and did not cause problems with
MIC, mg/L
1024 1 512 -
o*o 0
256 128 64 32 16 84-
210.5 0.25 -
0
0
I
I
I
I
0
I
Figure 1 Erythromycin, regression line for zone versus MIC (methods A and D). Asterix indicates 12 values. Regression line: y = - 1 2 . 7 6 ~+ 569,7; r2 = 0.87.
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respect to the overall purpose of the present study: the separation of isolates into susceptible and resistant groups. The proper separation of isolates is of major importance, as nalidixic acid is used to identify Cutnpylobucter to the species level [6], even though at present there is no zone diameter criterion for defining susceptibility to this agent. Resistance of C. jejuni and C. coli to quinolones is emerging in many countries [9-121. This will cause problems with species identification when this is performed solely by phenotypic tests, the most frequently used method for differentiating Cumpylobucter spp. in routine laboratories [13,14]. More noteworthy is the fact that nalidixic acid susceptibility is a marker for C. jejuni and C. coli ciprofloxacin susceptibility; the nalidixic acid-susceptible strains are susceptible to ciprofloxacin, while most of the resistant ones are resistant to ciprofloxacin [lo,151. A high level of nalidixic acid is therefore indicative of resistance to ciprofloxacin, an important drug of choice for the treatment of severe invasive campylobacteriosis [16]. The level of nalidixic acid resistance in C. jejuni isolated from humans in Denmark has previously been assessed by methods A and D, and is approximately Table 2 Tentative breakpoints for the four antimicrobial agents Antimicrobial
MIC methods
Tablet methods
(mg4
(mm)
SS16; R 2 3 2 SS8; R 2 6 4 S116; R S 3 2 S14; R 2 3 2
S226; R 1 1 7 S227; R 1 2 0 SS31; R 1 2 5 S225; R 1 1 8
agents Nalidixic acid Erythromycin Streptomycin Tetracycline
S, susceptible; R , resistant.
10-15% [17,18]. This moderate level can be attributed to the infrequent use of antibiotics for the treatment of diarrhea in Denmark and the fact that the veterinary use of quinolones for therapy of infections has so far been relatively limited. In the present study, all methods agreed in categorizing the naturally resistant C. luri isolate as resistant to nalidixic acid. The general tendency of the Etest to produce lower values than the agar dilution methods was more pronounced for erythromycin than for nalidixic acid. The tendency was mainly seen at very low MICs, and therefore did not interfere with the interpretation of results. Resistance to erythromycin is more frequent in C. coli than in C. jejuni [17,18]. In the present study, 42% of C. coli and 3% of C.jejuni isolates were resistant to erythromycin. The agent is considered to be the drug of first choice when simple rehydration and electrolyte replacement is not sufficient for the treatment of Campylobacter enteritis [16,19]. The reliability of the reported susceptibility testing on this agent is therefore of major clinical importance. Our regression line in Figure 1 indicates that the tablet method may constitute a simple, inexpensive and accurate means of susceptibility testing of Cumpylobucter for erythromycin. For streptomycin and tetracycline, there was complete agreement between all five methods. For both agents, a large number of isolates had MICs out of range, so the calculations in Table 3 are based upon moderate numbers, 16 and 25 respectively. As shown in Table 3 , method B resulted in lower values than methods A and C when tested for streptomycin. Interpretation was clearcut for tetracycline because of full agreement between methods and MICs and zones of inhibitions clearly away from the breakpoint. Correlation analysis with this antibiotic is weakened by
Table 3 Distribution of differencesb in MICs for susceptible isolates (%) Antimicrobial agent
~
a
N"
~~
Methods
< -2
-3
-1
0
+1
+2
>+2
~
N inmcates number of susceptible isolates for whch comparison was performed
Zero indlcates percentage of isolates for which MICs are identical within one ddutlon; - 1 and further.
+1 etc inmcate? 1 log2 ddution dlfference
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Clinical M i c r o b i o l o g y and Infection, Volume 5 N u m b e r 9 , September 1999
the low occurrence of resistance in the isolates in the study. The tetracyline-resistant C. jejuni reference strain was correctly categorized as resistant by all five methods, indicating that the low occurrence of resistance to tetracycline is correct and not methodological. Low levels of resistance to tetracycline in Denmark have previously been shown for both C. jejuni and C. coli [17,181. In conclusion, the Etest had a tendency to produce lower values compared to agar ddutions when tested against nahdixic acid and erythromycin. In contrast, method B resulted in lower values compared to the two other MIC methods when tested against streptomycin. However, both methods were able to separate isolates unequivocally into susceptible and resistant groups, and the findings are therefore of minor significance. Even though monitoring of the resistance for surveillance purposes is performed in different laboratories and with different techniques, our study indicates that reliable results can be achieved for comparison. However, the interpretive criteria that we have suggested must be considered tentative because of the low level of resistance to some antibiotics among a moderate number of isolates at study. It would be appropriate to evaluate these tentative criteria in a larger multicenter study, including a variety of antibiotics that have clinical, diagnostic or epidemiologic relevance. Acknowledgments
We are grateful to Annette Amdt, Frank Hansen, Bente Hansen, RenC Hendriksen and Anette Gggsig Christensen for technical assistance and Simoco ApS, Lyngby, Denmark for co-financing of Etest strips. This study was a part of the Danish Integrated Antimicrobial Resistance Monitoring and Research Program (DANMAP), conducted in collaboration between Statens Serum Institut, the Danish Veterinary and Food Administration and the Danish Veterinary Laboratory, and funded jointly by the Danish Ministry of Health and the Danish Ministry of Food, Agriculture, and Fisheries. References 1. Piddock LJV Quinolone resistance and Campylobacter spp. J Antimicrob Chemother 1995; 36: 891-98. 2. Gaunt PN, Piddock LJV Ciprofloxacin resistant Campylobactn spp. in humans: an epidemiological and laboratory study. J Antimicrob Chemother 1996; 37: 747-57. 3. World Health Orgdnization. The medical impact of the use of antimicrobials in food animals. Report of a WHO Meeting, Berlin.13-17 October 1997. Geneva: WHO, 1997.
4. World Health Organization.Major gaps in research on antibiotic resistance need filling. Press Release WH0/46, WHO, 1998. 5. Neimann J, Engberg J, M~lbakK, Wegener HC. Risk factors associated with sporadic campylobacreriosis in Denmark. In: Proceedings of the 4th World Congress on Foodborne Infections and Intoxications, Berlin: Federal Institute for Health Protection of Consumers and Veterinary Medicine, 1998: 29&303. 6. Barrow GI, Feltham RKA. Cowan and Steel's manual for the identification of medical bacteria, 3rd edn. Cambridge: Cambridge University Press, 1993. 7. Tenover FC, Baker CN, Fennell CL, Ryan CA. Antimicrobial resistance in Campylobacter species. In Nachamkin I, Blaser MJ, Thomkins LS, eds. Campylobacterjejuni: current status and future trends. Washington DC: American Society for Microbiology, 1992; 6G73. 8. National Committee for Clinical Laboratory Standards. Performance standards for antimicrobial disk susceptibility tests, 6th edn. Approved Standard. NCCLS document M2-M6. Wayne, PA. NCCLS, 1997. 9. Sjogren E, Lindblom GB, Kaijser B. Nodoxacin resistance in Campylobacter jejuni and Campylobacter coli isolates h m Swedish patients. J Antimicrob Chemother 1997; 4 0 257-61. 10. Rautelin H, Renkonen 0-V, Kosunen TU. Emergence of fluoroquinolone-resistance in Campylobacter jejuni and Campylobacter coli in subjects fkom Finland. Antimicrob Agents Chemother 1991; 35: 2065-9. 11. Sinchez R, Fernindez-Baca V, Diaz MD, Muiios P, RodriguezCreiems M, Bouza E. Evolution of susceptibilities of Campylobacter spp. to quinolones and macrolides. Antimicrob Agents Chemother 1994; 38: 1879-82. 12. Velizquez JB, Jimenez A, Chombn B, Villa TG. Incidence and transmission of antibiotic resistance in Campylobacter jejuni and Campylobacter coli. J Antimicrob Chemother 1995; 35: 173-8. 13 On SLW. Identification methods for campylobacters, helicobacters, and related organisms. Clin Microbiol Rev 1996; 9: 405-22. 14 Vandamme P, Goossens H. Taxonomy of Campylobacter, Arcobacter, and Helimbacter. a review. Z Bakteriol 1992; 276: 447-72. 15 Endtz PH, Ruijs GJ, van IUingern B, Jansen WH, van der Reyden T, Mouton RF! Quinolone resistance in Campylobacter isolated from man and poultry following the introduction of fluoroquinolones in veterinary medicine. J Antimicrob Chemother 1991; 27: 199-208. 16 Skirrow MB, Blaser MJ. Campylobatterjejuni. In Blaser MJ, Smith eds. Infections of PD, Ravdin JI. Greenberg HB, Guerrant IU, the gastrointestinal tract. New York: Raven Press, 1995: 825-48. 17. Aarestrup FM, Nielsen EM, Madsen M, Engberg J. Antimicrobial susceptibility patterns of thermophilic Campylobacter spp. &om humans, pigs, cattle and broilers in Denmark. Antimicrob Agents Chemother 1997; 41: 2244-50. 18. Anon. In: Consumption of antimicrobial agents and occurrence of antimicrobial resistance in bacteria fkom food animals, food and humans in Denmark, 1997. Danish Integrated Antimicrobial Resistance Monitoring and Research Programme (DANMAP), Copenhagen: Danish Zoonosis Center, 1998. 19. Taylor DE. Antimicrobial resistance of Campylobacter jejuni and Campylobacter coli to tetracycline, chloramphenicol and erythromycin. In Nachamkin I, Blaser MJ, Thomkins LS, eds. Campylobacter jejuni: current status and future trends. Washington DC: American Society for Microbiology, 1992 74-86.